1. Field of the Invention
The present invention relates to a drawing method, and more particularly to a drawing method for forming a reflective curved surface by pressing a metal plate and a die assembly which is suited to be used to carry out the drawing method.
2. Description of Related Art
In a printer which also has an image forming function by use of a laser beam, generally, a laser beam modulated in accordance with image data is imaged on a photosensitive member, and the modulation/imaging of a laser beam is carried out line by line. In this way, a two-dimensional image is formed.
In the image forming operation, before a start of modulation, the laser beam is incident to a photosensor which outputs an SOS (start of scanning) signal to time a start of writing of each line. For this purpose, conventionally, a structure as shown by FIG. 8a has been adopted, in which a laser beam L is reflected by a plane mirror 1 and converged on a sensor 3 by a lens 2. In the structure in which the plane mirror 1 and the lens 2 are used, the number of components is large, and the number of places to be adjusted is large. Therefore, this structure is not preferable.
In order to solve the problem, a structure as shown by FIG. 8b is adopted, in which a laser beam L is reflected and converged by a converging mirror 5. This mirror 5 has a reflective surface which also has an accurate converging function, and the reflective/converging surface is made by vapor-depositing a metal on a surface of a core which is a resin mold.
However, there are the following problems in forming the reflective/converging surface. It is very costly to form a coating on a surface of a resin mold by vapor deposition. The yield in the cycle of making resin molds and coating the resin molds by vapor deposition is bad, and it is always necessary to produce more than the necessity, which may result in too large a stock of products.
Meanwhile, Japanese Patent Laid-Open Publication No. 2002-316226 discloses a method of forming a reflective optical element from a metal plate by drawing. By this method, it is possible to form an optical element at low cost. However, there are still problems. The reflective surface made by this method is not very accurate due to spring kick at a bent portion. A complex die is necessary, and the productivity is low.